1. Field of the Invention
The present invention concerns a process and an apparatus for grinding tyres.
More particularly, the invention has as its subject a process for grinding tyres carried out to improve the dynamic behaviour of the tyre itself.
2. Description of the Related Art
A tyre for vehicle wheels generally includes a carcass structure including at least one carcass ply having respectively opposed boundary edges secured to respective annular anchoring structures, built into the areas usually identified with the name of “beads”, having an inner diameter substantially corresponding to a so-called “seating diameter” of the tyre on a respective rim.
The carcass structure is associated with a belt structure including one or more layers of belt located in radial superimposition one with respect to another and with respect to the carcass ply, having textile or metal reinforcing cords with orientation crossed and/or substantially parallel to the direction of circumferential extension of the tyre. In a position radially external to the belt structure a tread band is applied, this too being in elastomeric material like other semifinished constituent parts of the tyre.
On the lateral surfaces of the carcass structure, each extending from one of the lateral edges of the tread band to the vicinity of the respective annular structures anchoring the bead, there are respective sidewalls in elastomeric material applied in an axially external position. On tyres of “tubeless” type, a layer of coating impermeable to air, usually called a “liner”, covers the inner surfaces of the tyre.
The process of making up the raw tyre, which is performed by assembling the respective components, is generally followed by carrying out a moulding and vulcanisation treatment aimed at bringing about the structural stabilisation of the tyre by reticulation of the elastomeric components, and also a process of impressing on it a desired tread pattern and any distinctive graphic marks in the region of the sidewalls of the tyre.
After vulcanisation, the tyres are subjected to a visual check, to identify any external geometrical irregularities, particularly on the tread, and a dynamic check, carried out while the tyre is revolving, to identify any imbalances due to defects internal to the structure, situated for example in the vicinity of the shoulders and/or the sidewalls of the tyre.
Tyres which do not fall within predetermined parameters with regard to geometrical and/or dynamic defects, and which therefore ought to be scrapped, undergo further processing to improve both the above-mentioned aspects. These processes enable a proportion of the defects to be remedied and bring about a reduction in the percentage of total tyre production which has to be scrapped. Document U.S. Pat. No. 3,491,493 describes a method and an apparatus for improving the dynamic behaviour of a tyre by abrasive removal of a portion of the external surface of the tyre itself. The size and the position are determined of the maximum radial force exercised by the tyre in rotation under load. The tyre is mounted eccentrically, in such a way that the maximum eccentricity coincides with the point of maximum radial force, and the tyre is rotated against a grinding wheel which removes material from the tyre so as to compensate for the irregularities in the tyre and thus produce a tyre with more regular behaviour on the road.
Document U.S. Pat. No. 3,724,137 discloses the combination of a grinder and a control system including means of supporting a tyre while it revolves around its own axis. Grinding wheels are mounted close to the tyre and are movable with respect to the tyre itself to grind selected portions of the same tyre at rest in proportion to variations in force measured around the circumference of the tyre. The control system includes means of generating electrical signals representing the variations in force and including transducers engaging the tyre, a memory for storing the electrical signals and emitting an output signal in response to the information stored, a servo mechanism for moving the grinding wheels with respect to the tyre, from and towards a grinding position in response to the output signal just mentioned.
Document U.S. Pat. No. 3,739,533 describes a method of optimizing uniformity for a tyre which includes the stage of measuring the deviation in the lateral force and the variation in the radial force of a tyre set in rotation, of generating corrective signals on the basis of the forces measured, of generating from the corrective signals two composite signals and of grinding the edges of the tread of the tyre in response to the composite signals.
Document U.S. Pat. No. 3,817,003 discloses a programmable system for increasing the capability of grinding machines by incorporating programmed switching from grinding based on the actual variation in force to grinding based on a harmonic of the variation in force.
Document U.S. Pat. No. 4,173,850 discloses a method and an apparatus for reducing the variation in tangential force in a tyre. The tyre is mounted on a rim and made to rotate against a loading drum. Force transducers located on the axis of the drum measure the variation in tangential force while the wheel rotates. The measurement is stored in the memory of a calculator which processes the data and obtains the first harmonic of this force, the magnitude of this first harmonic and the angular distance from the maximum point of this magnitude. The grinding wheel engages the tyre from a point 90° before this maximum to a point 90° after the maximum. The shoulders of the tyre are not touched by the grinder unless grinding is being done to reduce the variation in lateral and radial forces.